He already has two boards that measure 9 feet and 12 feet. Select all of the values that could represent the length of the third board of his triangular sandbox.

Picture containing the options is attached below

9 feets, 15 feets, 19 feets

Explanation:

15, 30, 21

Given that the length of two of the 3 sides aee:

9 feets and 12 feets.

If the 3 sides are a, b and c

a = 9 feets ; b = 12 feets

The length of side c, could be any value between the difference of the two Given sides or the sum, Thus ;

(a - b) < c < (a + b)

(12 - 9) < c < (12 + 9)

3 < c < 21

Hence, c coukd be values between 3 and 21

9 feets, 15 feets, 19 feets

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